1. Field of the Invention
The present invention relates to a mixer, and more particularly, to a mixer for down-converting a radio frequency (RF) input signal into a low intermediate frequency (low-IF) output signal or a zero intermediate frequency (zero-IF) output signal.
2. Description of the Prior Art
Mixers in conventional signal receiving systems can be implemented by using at least one Gilbert cell. Please refer to FIG. 1A. FIG. 1A is a diagram of a conventional mixer 100 implemented by using a single Gilbert cell. As shown in FIG. 1A, the mixer 100 comprises an amplifying circuit 105 and a down-converting circuit 110. The amplifying circuit 105 includes a transistor Q1, which functions as a transconductance stage in the mixer 100, for amplifying an RF input signal Sin to generate an amplified signal Sin′. The down-converting circuit 110 includes a plurality of resistors R1, R2 and a plurality of transistors Q2, Q3. A switching stage in the mixer 100 is formed by the transistors Q2, Q3 and a loading stage in the mixer 100 is made up of the resistors R1, R2. The down-converting circuit 110 is also a differential circuit for generating an output signal Sout according to a local oscillating signal SLO. This output signal Sout can be a low-IF output signal or a zero-IF output signal. Additionally, the amplifying circuit 105 is also used as a low noise amplifier (LNA) and coupled to the down-converting circuit 110. Some problems may arise, however, when performing a down-converting operation by using the mixer 100 to generate the output signal Sout.
First of all, an input impedance of an antenna is often 50 Ohms or 75 Ohms, but the impedance of the mixer 100 as seen from the gate of the transistor Q1 (i.e. the input impedance of the mixer 100) is usually far higher than that of the antenna. For instance, the input impedance of the mixer 100 could be even higher than 1000 Ohms. Thus, it is necessary to add an impedance matching circuit between the antenna and the mixer 100 for solving the impedance mismatch problem when using the mixer 100 to perform down-converting operations. However, the total circuit area will be significantly increased due to this impedance matching circuit. Even though the configuration (i.e. a common-source configuration) of the transistor Q1 in the amplifying circuit 105 may be changed into a common-gate configuration in another example, the total circuit area is still increased due to this required impedance matching circuit.
Secondly, according to mixer theory, the low-frequency noise in the output signal Sout depends on DC currents passing through the transistors Q2 and Q3 in the down-converting circuit 110. In addition to performing the down-converting operation on the amplified signal Sin′ to generate the output signal Sout, the down-converting circuit 110 is further utilized for providing a DC current to the amplifying circuit 105. The noise figure of the mixer 100 will therefore be higher. In particular, when the above-mentioned transistors are implemented by using complementary metal-oxide-semiconductor field-effect transistors (CMOSFETs), the noise figure becomes much higher due to the flicker noise from the CMOSFETs.
As mentioned above, the conventional mixer can also be implemented by using a pair or a plurality of Gilbert cells. A mixer composed of a pair of Gilbert cells is referred as to a double balance mixer. Please refer to FIG. 1B. FIG. 1B is a diagram of a conventional double balance mixer 115. As shown in FIG. 1B, the mixer 115 includes an amplifying circuit 120, a down-converting circuit 125, and a switching circuit 130. The mixer 115 is utilized for performing the down-converting operation on an RF input signal Sin to generate differential output signals Sout+ and Sout− according to local oscillating signals SLO+ and SLO−, where the voltage levels Vbias and Vbias′ shown in FIG. 1B are used for biasing. The switching circuit 130 is utilized for extracting a DC current from the Voltage Source VCC and providing the amplifying circuit 120 with the extracted DC current, to reduce the problem caused by the higher noise figure. Although the design of the switching circuit 130 can reduce DC currents passing through the down-converting circuit 125, there may still exist a few DC currents passing through the transistors in the down-converting circuit 125. With respect to the mixer 115, the DC currents passing through the transistors in the down-converting circuit 125 are smaller than those in the down-converting circuit 110 of the mixer 100; however, the existence of such DC currents means that the noise figure of the mixer 115 is still very high.
As well as active mixers, the conventional mixer can also be implemented by utilizing a passive mixer. Please refer to FIG. 1C. FIG. 1C is a diagram of a conventional passive mixer 135. As shown in FIG. 1C, the mixer 135 performs the down-converting operation on RF input signals Sin+ and Sin− to generate output signals Sout+ and Sout− according to local oscillating signals SLO+ and SLO−. Since no DC current passes through transistors Q1, Q2, Q3, and Q4 of a switching stage in the mixer 135 (i.e. no large electric field exists between the drains and sources of the transistors Q1˜Q4 almost), the noise figure of the mixer 135 can be lower than that of the active mixer. However, the ability of the mixer 135 to amplify signals is much worse than that of the active mixer. Additionally, output ends of the switching stage in the mixer 135 are usually connected to input ends of the next stage circuit after performing the down-converting operation, and the impedance at the input ends of the next stage circuit often comprises a high real part when the mixer 135 operates at a very low frequency. The impedance seen at the input ends of the mixer 135 therefore becomes much higher. Thus, a serious impedance mismatch problem arises, which leads to considerable signal degradation if the impedance at input ends of the mixer 135 as seen from output ends of a previous stage circuit (compared with the mixer 135) is much higher. Even though the problem resulting from a higher noise figure of the active mixer can be solved by using the passive mixer to replace the active mixer, the impedance mismatch problem and considerable signal degradation can never be solved. Moreover, an impedance matching circuit between the antenna and the input end of the passive mixer is always required